Do I need to look at anything in System Preferences? Sharing?
The only System Preferences items that you should double-check, is on the Network pane.
I've included screenshots from my Mac mini to compare. (Note: My mini is running Lion so it may look a bit different than yours.)
Two items, in particular, will be important:
The Network Connector order. Ethernet (or Built-in Ethernet) should be at the top of the list.

Click on the Advanced button, and then, on the TCP/IP tab, check that the option: Configure IPv4: Using DHCP. You can also use the Renew DHCP Lease option to see if after clicking on it, you now get the proper IP addresses.
Finally, on the Hardware tab, the default is option: Configure: Automatically. Which should work 99.99% of the time, but you may want to experiement and try (at least) the following by switching to Manually:
I would recommend leaving Duplex set to: full-duplex, flow-control, and MTU set to: Standard (1500)
